Variable frequency control circuit

ABSTRACT

A variable frequency pulse generator is employed to control the frequency of a monostable multivibrator which provides drive pulses of a constant pulse width to a D.C. load. Discharge circuits are provided to discharge a capacitor in the pulse generator during the operation of the multivibrator and to drive down the charge in a multivibrator capacitor at the end of each drive pulse to provide substantially instantaneous recycling.

United States Patent Hoge [54] VARIABLE FREQUENCY CONTROL CIRCUIT [72]Inventor: Henri H. Hoge, Baltimore, Md. [73] Assignee: Rhomega System,Incorporated [22] Filed: April 17, 1970 [21] App1.No.: 29,494

521 u.s.c|. ..307/271, 307/265,318/341 51 Int. Cl. ..H03k 1/10 58FieldofSearch ..307/265, 271;31s/13s, 341,

[56] References Cited 1 UNITED STATES PATENTS 3,358,206 12/1967 Thiele..318/341 3,260,853 7/1966 Kihara ..307/27l 3,446,992 5/1969 Webb..307/271 151 3,681,620 [451 Aug. 1,1972

Van Eyk ..317/l37 3,191,113 6/1965 Gargani ..318/341 3,214,666 10/1965Clerc ..318/341 3,223,912 12/1965 Sheheen ..318/341 3,378,745 4/1968James ..318/341 Primary Examiner-Donald D. Forrer Assistant Examiner-R.E. Hart Attorney-Colton & Stone [5 7] ABSTRACT A variable frequencypulse generator is employed to control the frequency of a monostablemultivibrator which provides drive pulses of a constant pulse width to aDC. load. Discharge circuits are provided to discharge a capacitor inthe pulse generator during the operation of the multivibrator and todrive down the charge in a multivibrator capacitor at the end of eachdrive pulse to provide substantially instantaneous recycling.

9 Claims, 1 Drawing Figure PATENT-Ems 1 I972 INVENTOR HENRI H. HOGEcammkz VARIABLE FREQUENCY CONTROL CIRCUIT BACKGROUND OF THE INVENTIONThe present invention relates generally to control circuitry operable tocontrol an integrating load powered from a D.C. supply source and moreparticularly to a solid state speed control circuit having substantiallyinstantaneous recycle characteristics which is particularly adapted forD.C. motor control.

An increasing demand for portable or compact electrical units poweredfrom batteries or similar D.C.

supply sources of limited voltage has led to the development of solidstate control circuits for various D.C. load devices. Such circuits areprevalent in the D.C. motor control field, for a D.C. motor is a commondrive source for compact electrical units. These motors are found inwindshield wipers, automobile airconditioners, blowers and heaters,battery driven vehicles, power'tools and appliances, andin many similarcompact or portable units.

Considering a D.C. motor as an exemplary load, it must be noted thatsuch motors are designed to strict tolerances and are adapted to providea set rate of speed for a specific level of input voltage. In the past,speed control units for D.C. motors have been developed which controlmotor speed by converting an input from a D.C. power source into apulsed D.C. signal which is employed to drive the motor. To vary motorspeed, the pulse width of the motor driving pulses is varied. Thesevariable pulse width speed control systems suffer from a number ofdisadvantages. For example, the inertia of the motor armature must beovercome if the motor is to operate, and with pulse width controlsystems, motor armature inertia may not be effectively overcome duringshorter pulse time control periods. This results in erratic motoroperation.

In an attempt to alleviate the problems resulting from variable pulsewidth control systems, variable frequency motor control systems havebeen developed wherein the frequency of the motor drive pulses is variedto alter motor speed. Often, as illustrated by US. Pat. No. 3,446,992 toJames E. Webb, such variable frequency control systems also incorporatevariable pulse width control. As will be noted from the Webb patent,variable frequency motor control may be achieved by employing anoscillator type pulse generator with a multivibrator to vary thefrequency of motor drive pulses derived from a D.C. power source.However, the pulse generators and multivibrators of these speed controlcircuits employ capacitors which must be charged and discharged toprovide each cycle of the motor drive signal. The recycle time of thesedevices is extremely slow, and severely limits the maximum speedattainable with a D.C. motor employing a variable frequency speedcontrol unit. Often, the speed control unit will reduce by as much as 50percent the maximum speed of a D.C. motor at a rated voltage input. Thusa D.C. motor rated by the manufacturer to provide 200 rpm at 14 voltsD.C. may only provide 100 rpm at 14 volts D.C. when a conventionalvariable frequency speed control unit is interposed between the D.C.source and the motor.

It is a primary object of the present invention to provide a novel andimproved variable frequency control unit operable from a D.C. source toprovide variable frequency drive pulses of constant pulse width to aD.C. load.

Another object of the present invention is to provide a novel andimproved solid state variable frequency control unit operable from aD.C. source to provide variable frequency drive pulses of constant pulsewidth to a load which is adapted to recycle substantiallyinstantaneously.

A still further object of the present invention is to provide a noveland improved solid state variable frequency control unit particularlyadapted. for D.C. motor speed control. This unit provides pulses ofconstant pulse width to a controlled D.C. motor through the action of amonostable multivibrator which is driven by a pulse generator. Both themultivibrator and pulse generator are provided with capacitor dischargecircuits which permit substantially instantaneous recycling of thecontrol unit.

These and other objects of the present invention will become apparentupon a consideration of the following specifications taken inconjunction with the accompanying drawing which illustrates a circuitdiagram of the variable frequency control circuit of the presentinvention.

Referring now to the drawing, the control circuit of the presentinvention indicated generally at 10 includes a variable frequency pulsegenerator 12 and a monostable multivibrator 14 connected to anintegrating D.C. load; in this case a D.C. motor 16. The variablefrequency pulse generator includes a voltage divider formed by resistors18 and 20 connected in series across D.C. input terminals which may beconnected to any suitable D.C. voltage source. A positive line 22connected to the positive terminal of the D.C. source provides power tothe control circuit 10, and a return line 24 is connected to thenegative terminal of the D.C. source.

a resistor 28 and a capacitor 30 is also connected across the inputterminals in parallel relationship to the voltage divider. The variablefrequency pulse generator is completed by a transistor 32 having acollector electrode 34, an emitter electrode 36 connected between theresistors 18 and 20, and a base electrode 38 connected between theresistor 28 and capacitor 30.

The monostable multivibrator 14 includes a transistor 40 having anemitter electrode 42 connected to the line 22, a base electrode 44connected to the collector 34 of the transistor 32 and to the line 22through a base resistor 46, and a collector electrode 48 connected inseries with resistors 50 and 52 to the terminal 24. A second transistor54 includes a base electrode 56 connected between the resistors 50 and52, an emitter electrode 58 connected to the line 24 and a collectorelectrode 60 connected to the base electrode 62 of a third transistor64. The third transistor also includes a collector electrode 66connected to the emitter electrode 58 of the transistor 54 and anemitter electrode 68connected by means of a resistor 70 to the collectorelectrode 60. The motor 16 is connected between the emitter electrode 68and the input line 22.

The on time of the monostable multivibrator 14 is determined by a seriesRC circuit including a capacitor 72 connected to a point between themotor 16 and the emitter electrode 68 and a resistor 74 connected to thebase 44 of the transistor 40 and the collector 34 of the transistor 38.A diode 76 having an anode connected between the capacitor 72 and theresistor 74 and a cathode connected to the input line 22 provides adischarge path for the capacitor 72.

The capacitor 30 is discharged during the operation of the monostablemultivibrator 14 by a diode 78 having an anode connected between theresistor 28 and the capacitor 30 and a cathode connected between themotor 16 and the emitter 68.

When the D.C. motor 16 forms the load for the control circuit 10, a freewheeling diode 80 is connected across the motor to provide a dischargepath for reactive energy stored in the motor when the monostablemultivibrator 14 is active.

In the operation of the control circuit 10, the monostable multivibrator14, which includes complementary transistor sections, is normally off.When the control circuit is connected to the D.C. power supply, such as,for example, by the operation of a switch 51, the capacitor 30 begins tocharge through potentiometer 26 and resistor 28. Preferably, thepotentiometer 26 is a right hand logarithmic potentiometer whichoperates to vary the charging time of the capacitor 30 and thus thefrequency of the output pulses from the pulse generator 12 and the speedof the motor 16. The

motor speed will be a linear function of the angular rotation of thepotentiometer 26 when a logarithmic potentiometer is employed.

The resistor 28 limits the maximum charging speed for the capacitor 30and therefore limits the number of pulses generated by the pulsegenerator 12 in any time period. Also, the resistors 18 and 20 hold theemitter electrode 36 of the transistor 32 at a set D.C. level, and thecapacitor 30 must charge above this emitter voltage level before thetransistor 32 can conduct.

When the capacitor 30 raises the'base voltage of the transistor 32 abovethe emitter voltage level set by the resistors 18 and 20, the transistorconducts causing the current to the collector 34 thereof through theresistor 46 to form the base current for the transistor 42 of themonostable multivibrator 14. This transistor now begins to conduct, andcurrent from the collector 48 thereof flows through the resistors 50 and52 to drive the transistors 54 and 64 into conduction. This results inthe application of power to the motor 16.

The conduction of the transistors 54 and 64 causes the plate of thecapacitor 72 connected thereto to be pulled down, and the capacitorbegins to charge through the resistor 74, thus driving the transistor tosaturation. Also, conduction of the transistors 54 and 64 causes thediode 78 to conduct to discharge the capacitor 30. Thus this capacitoris put in condition for a new cycle of operation during the operation ofthe monostable multivibrator 14.

The capacitor 72 will continue to charge until a charge level is reachedwhere the current through the resistor 74 begins to decrease andtransistor 40 comes out of saturation. Current through the resistor 74will continue to decrease as the capacitor 72 attains a maximum chargecapacity, and current through the transistor 40 to the base of thetransistor 54 will decrease. This results in a decreased conduction ofthe transistors 54 and 64, and the plate of the capacitor 72 connectedthereto will now be driven positive. This reverses the current to thetransistor 40 by way of the resistor 74, and the transistors 40, 54 and64 are driven off. Also the charge on the capacitor 72 is driven downthrough the diode 76, through the motor 16 and back to the negative sideof the capacitor. This actual driving of the charge from the capacitor72 through the diode 76 is extremely important, for the discharge of thecapacitor is substantially instantaneous. The capacitor 30 haspreviously been discharged, so the recycle time of the control circuit10 is limited only by the discharge time for the capacitor 72. As thisdischarge is practically instantaneous, the control circuit recycle timeis also substantially instantaneous. In practice, it has been found thatthe control circuit 10 delivers up to 98 percent of the rated motorspeed of a D.C. motor.

The on time of the monostable multivibrator l4 and thus the pulse widthof the driving pulses to the motor 16 is determined by the resistor 74and the capacitor 72, and is maintained at a constant value. In the caseof an electric motor control, this resistor and capacitor are matched tothe particular electric motor employed so that the drive pulse width isalways sufiicient to overcome the inertia of the motor armature. Also,the diode 76 operates to stabilize the pulse width set by the resistor74 and the capacitor 72, for the diode starts to discharge the capacitordown to a specific reference level the moment the multivibrator 14begins to shut off. This stable reference level is maintained by thediode 76 so that each drive pulse begins at the same point on the chargecurve for the capacitor 72.

The control circuit 10 is particularly well adapted for D.C. motor speedcontrol but it will be readily apparent to one skilled in the art thatthe extremely rapid recycle capability of this circuit facilitatesadaptation of the circuit as a control for a variety of integrating D.C.loads.

Iclaim:

1. A control circuit for providing drive pulses of variable frequencyand constant pulsewidth from a D.C. power source to a load comprisingvariable frequency pulse generator means operative to provide outputpulses of a selected frequency and drive pulse switching meansconnected, to receive said output pulses and operative in responsethereto to pass drive pulses of constant pulse width through said loadfrom said D.C. power source, said pulse switching means operating tovary the frequency of said drive pulses in response to variations in thefrequency of said output pulses.

2. The control circuit of claim 1 wherein said variable frequency pulsegenerator means includes a capacitor, a variable charging circuitconnected between said D.C. source and said capacitor for charging saidcapacitor at a preselected rate, and semiconductor means connected tofire and provide an output pulse when said capacitor reaches adetermined charging point, and capacitor discharge means connected tosaid capacitor and to said drive pulse switching means, said capacitordischarge means being operative by said drive pulse switching means uponreceipt thereby of an output pulse to discharge said capacitor.

3. The control circuit of claim 1 wherein said drive pulse switchingmeans includes a monostable multivibrator having first transistor meansconnected to receive said output pulses, said first transistor meansbeing rendered conductive upon receipt thereby of an output pulse,second transistor means connected to be rendered conductive uponconduction of said first transistor means, and multivibrator capacitormeans connected between said first and second transistor means andoperative upon conduction of said second transistor means to charge toincrease the conduction of transistor means, said multivibratorcapacitor means operating upon reaching a determined charge to decreasethe conduction of said first transistor means until termination ofconduction thereby.

4. The control circuit of claim 3 wherein a multivibrator capacitordischarge means is connected to said multivibrator capacitor means, saidsecond transistor means being operated by the decrease in conduction ofsaid first capacitor means to decrease conduction of said secondtransistor means until termination of conduction thereof, said secondtransistor means operating subsequent to the initiation ofdecreasedconduction therethrough to cause said multivibrator capacitorto discharge through said multivibrator capacitor discharge means.

5. The control circuit of claim 4 wherein said multivibrator capacitordischarge means includes diode means operative to discharge saidmultivibrator capacitor means to a set discharge level.

6. The control circuit of claim 4 wherein said second transistor meansoperates to reverse bias said multivibrator capacitor means to rapidlydischarge said multivibrator capacitor means through said multivibratorcapacitor discharge means.

7. The control circuit of claim 6 wherein said variable frequency pulsegenerator means includes a capacitor, a variable charging circuitconnected between said D.C. source and said capacitor for charging saidcapacitor at a preselected rate, and semiconductor means connected tofire and provide an output pulse when said capacitor reaches adetermined charging point, and capacitor discharge means connected tosaid capacitor and to said drive pulse switching means, said capacitordischarge means being operative by said drive pulse switching means uponreceipt thereby of an output pulse to discharge said capacitor.

8. The control circuit of claim 7 wherein said variable charging circuitincludes a logarithmic potentiometer 9. The control circuit of claim?wherein said capacitor discharge means includes a diode having an anodeconnected to said capacitor and a cathode connected to said secondtransistor means, said diode being biased into conduction upon theconduction of said second transistor means.

I UNITED STATES PATENT OFFICE CERTIFICATE OF CGRRECTEON Patent No;3.681.620 Dated Auqust 1, 19

In ent r( HENRI H. HOGE It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

insert.-said first.

Signed and sealed this 9th day of January 1973.

(SEAL) Attest:

EDWARD M.FLETCH3R,JR. Attesting Officer ROBERT GOTTSCHALK Commissionerof Patents UNITED STATES Q PATENT OFFICE CERTIFICATE OF CORRECTIONPatent No. 3, 5 1, 20 Dated Auqust l, 1972 lnvento' -(s) H.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 5, line 4, after "of" and before 'transistor",

insert -said first.

(SEAL) Attest:

EDWARD M.FIETCHER,JR. Attesting Officer ROBERT GOTTSCHALK Commissionerof Patents

1. A control circuit for providing drive pulses of variable frequencyand constant pulsewidth from a D.C. power source to a load comprisingvariable frequency pulse generator means operative to provide outputpulses of a selected frequency and drive pulse switching means connectedto receive said output pulses and operative in response thereto to passdrive pulses of constant pulse width through said load from said D.C.power source, said pulse switching means operating to vary the frequencyof said drive pulses in response to variations in the frequency of saidoutput pulses.
 2. The control circuit of claim 1 wherein said variablefrequency pulse generator means includes a capacitor, a variablecharging circuit connected between said D.C. source and said capacitorfor charging said capacitor at a preselected rate, and semiconductormeans connected to fire and provide an output pulse when said capacitorreaches a determined charging point, and capacitor discharge meansconnected to said capacitor and to said drive pulse switching means,said capacitor discharge means being operative by said drive pulseswitching means upon receipt thereby of an output pulse to dischargesaid capacitor.
 3. The control circuit of claim 1 wherein said drivepulse switching means includes a monostable multivibrator having firsttransistor means connected to receive said output pulses, said firsttransistor means being rendered conductive upon receipt thereby of anoutput pulse, second transistor means connected to be renderedconductive upon conduction of said first transistor means, andmultivibrator capacitor means connected between said first and secondtransistor means and operative upon conduction of said second transistormeans to charge to increase the conduction of transistor means, saidmultivibrator capacitor means operating upon reaching a determinedcharge to decrease the conduction of said first transistor means untiltermination of conduction thereby.
 4. The control circuit of claim 3wherein a multivibrator capacitor discharge means is connected to saidmultivibrator capacitor means, said second transistor means beingoperated by the decrease in conduction of said first capacitor means todecrease conduction of said second transistor means until termination ofconduction thereof, said second transistor means operating subsequent tothe initiation of decreased conduction therethrough to cause saidmultivibrator capacitor to discharge through said multivibratorcapacitor discharge means.
 5. The control circuit of claim 4 whereinsaid multivibrator capacitor discharge means includes diode meansoperative to discharge said multivibrator capacitor means to a setdischarge level.
 6. The control circuit of claim 4 wherein said secondtransistor means operates to reverse bias said multivibrator capacitormeans to rapidly discharge said multivibrator capacitor means throughsaId multivibrator capacitor discharge means.
 7. The control circuit ofclaim 6 wherein said variable frequency pulse generator means includes acapacitor, a variable charging circuit connected between said D.C.source and said capacitor for charging said capacitor at a preselectedrate, and semiconductor means connected to fire and provide an outputpulse when said capacitor reaches a determined charging point, andcapacitor discharge means connected to said capacitor and to said drivepulse switching means, said capacitor discharge means being operative bysaid drive pulse switching means upon receipt thereby of an output pulseto discharge said capacitor.
 8. The control circuit of claim 7 whereinsaid variable charging circuit includes a logarithmic potentiometer 9.The control circuit of claim 7 wherein said capacitor discharge meansincludes a diode having an anode connected to said capacitor and acathode connected to said second transistor means, said diode beingbiased into conduction upon the conduction of said second transistormeans.